Although the majority of transistors in most integrated circuits are CMOS transistors, bipolar transistors may also be constructed in a complementary metal-oxide-semiconductor (CMOS) integrated circuit to take advantage of areas where bipolar transistors outperform MOS transistors. For example bipolar transistors have higher transconductance, higher output impedance and faster switching speed. Vertical bipolar transistors which may be formed when manufacturing a CMOS integrated circuit with no extra cost have the ability to sink larger currents per unit device area than MOS transistors.
In addition, vertical bipolar transistors may be useful as a linear circuit such as a temperature stable voltage regulator, a bandgap reference circuit, a low input offset circuit, and a feedback amplifier circuit.
When building vertical bipolar transistor using a CMOS integrated circuit manufacturing flow it is difficult to optimize bipolar transistor characteristics such as bipolar transistor gain (hfe) without incurring additional processing cost.